Reset CUDAFatAgent _nextID when purging agent populations.

Mostly so submodels always reset agent id for internal pops.

include/flamegpu/gpu/CUDAFatAgent.h

@@ -159,6 +159,12 @@ class CUDAFatAgent {
      * Resets the flag agent_ids_have_init
      */
     void markIDsUnset() { agent_ids_have_init = false; }
+    /**
+     * Resets the ID counter (_nextID) to the first valid ID
+     * Useful for submodels which will keep resetting an agent population
+     * @note This will fail silently if it called if any state contains agents
+     */
+    void resetIDCounter();
 
  private:
     /**
@@ -198,7 +204,7 @@ class CUDAFatAgent {
     unsigned int mappedAgentCount;
     /**
      * Holds the next ID to be assigned
-     * IDs are assigned in consecutive order, however it cannot be guarnteed that IDs will always be consecutive due to agent death, parallel birth and multiple states.
+     * IDs are assigned in consecutive order, however it cannot be guaranteed that IDs will always be consecutive due to agent death, parallel birth and multiple states.
      */
     id_t _nextID;
     /**

src/flamegpu/gpu/CUDAAgent.cu

@@ -704,16 +704,21 @@ void CUDAAgent::initExcludedVars(const std::string &state, const unsigned int&co
     sm->second->initExcludedVars(count, offset, scatter, streamId, stream);
 }
 void CUDAAgent::cullUnmappedStates() {
+    int i = 0;
     for (auto &s : state_map) {
         if (!s.second->getIsSubStatelist()) {
             s.second->clear();
+            ++i;
         }
     }
+    if (i == state_map.size())
+        fat_agent->resetIDCounter();
 }
 void CUDAAgent::cullAllStates() {
     for (auto &s : state_map) {
         s.second->clear();
     }
+    fat_agent->resetIDCounter();
 }
 std::list<std::shared_ptr<VariableBuffer>> CUDAAgent::getUnboundVariableBuffers(const std::string& state) {
     const auto& sm = state_map.find(state);

src/flamegpu/gpu/CUDAFatAgent.cu

@@ -15,7 +15,7 @@ CUDAFatAgent::CUDAFatAgent(const AgentData& description)
     : mappedAgentCount(0)
     , _nextID(ID_NOT_SET + 1)
     , d_nextID(nullptr)
-    , hd_nextID(0) {
+    , hd_nextID(ID_NOT_SET) {
     for (const std::string &s : description.states) {
         // allocate memory for each state list by creating a new Agent State List
         AgentState state = {mappedAgentCount, s};
@@ -381,3 +381,10 @@ void CUDAFatAgent::assignIDs() {
 
     agent_ids_have_init = true;
 }
+void CUDAFatAgent::resetIDCounter() {
+    // Resetting ID whilst agents exist is a bad idea, so fail silently
+    for (auto& s : states_unique)
+        if (s->getSize())
+            return;
+    _nextID = ID_NOT_SET + 1;
+}